1. Field of the Invention
This invention relates to heat exchangers that have fins which are provided with louvers.
2. Background Art
Heat exchangers are used in air conditioners and heat pumps to transfer energy between two fluid media, e.g., a refrigerant fluid and air. The refrigerant fluid is circulated through relatively small diameter tubes and air is passed over the surface of the tubes so that heat may be transferred to the refrigerant fluid through the material of the heat exchanger tube from the ambient air. Thin metal sheets, or fins, can be attached to the heat exchanger tubes, thus presenting a larger surface area in contact with the air and thereby enhance heat transfer. The fins may include apertures that receive the tubes. The fins are securely held in thermal contact with the tubes. By the forced convection caused by a fan system, heat is transferred between the fin material and the circulating air. By thermal contact with the tubes, the fins conduct heat between the externally circulating air and the refrigerant fluid in the heat exchanger tubes.
Finned tube heat exchangers are widely used in a variety of applications, including refrigeration, air-conditioning and the like. Such heat exchangers have spaced parallel tubes in which a first heat transfer fluid such as water, oil, air or a refrigerant flows while a second heat transfer fluid such as air is directed across the outside of the tubes. Usually multiple fins are arranged in a parallel relationship between the tubes to form multiple paths for the second heat transfer fluid to flow across the fins and around the tubes.
Fin design is one factor in determining the heat transfer efficiency of a heat exchanger. Numerous fin designs have been proposed in the prior art to enhance heat transfer efficiency, compactness and manufacturability of finned tube heat exchangers. Many of these designs involve enhancements to the fins such as interrupting the fins with louvers that cause disruption of hydrodynamic boundary layers which form with increasing thickness along the fins and decrease heat transfer efficiency. Typically, such louvers are formed by first cutting the fin sheet at selected locations and then in a separate operation punching the fin material to form the louvers. Examples of prior art louvered fin heat exchangers are disclosed in U.S. Pat. Nos. 4,723,599 and 5,042,576.
Although louvered fins are advantageous from a heat transfer efficiency standpoint, the formation of the louvers adds complexity to the manufacturing process because two additional steps are typically involved—cutting the fin material and then pushing the material up or down to form the louvers. Further, formation of the louvers increases mechanical stresses on the fin sheets, which can cause deformation of the fins and other problems during the manufacturing process.
In general, prior art fin louvered geometries tend to become air side pressure limited as the fin density increases due to inadequate airflow passages. Other things being equal, the number of fins in contact with refrigerant tubes is a primary determinant of the thermal hydraulic efficiency of heat exchangers. It would be desirable to address problems of ineffective and suboptimal louver alignments between fins that are evident in some prior art approaches.
U.S. Pat. No. 5,509,469 discloses a flat fin heat exchanger with louvers and a rib that is raised above the plane of the fin that connects adjacent tube collars. The raised rib is said to enhance the heat transfer characteristics of the fin while allowing thinner materials to be used, thereby lowering costs without diminishing performance.
U.S. Pat. No. 5,752,567 discloses reinforcing ribs (22), longitudinally spaced flat portions (24,24a) and a central corrugated portion (26).
U.S. Pat. No. 5,730,214 discloses a heat exchanger with a corrugated cooling fin (FIG. 6) that has a louver pattern in which the louvers in a first lead set successively rise in tilt angle in the direction of airflow. Matching air louvers in a trailing set successively decrease in tilt angle.
U.S. Pat. No. 6,805,193 discloses a fin louver arrangement that uses breaking and reversal louvers, the lengths of which are substantially longer than the half length of the main louver but at slightly lower angles to the fin face.
U.S. Pat. No. 4,434,844 discloses a cross-fin coil type heat exchanger with raised fins and louvers that are formed on the surface of a convoluted fin base plate.
Conventional wisdom concerning louvered fin performance is derived from the channeling of airflow over the fin surfaces. At high air velocities, a measure of air turbulence is obtained thru the Reynolds number, the magnitude of which determines whether the airflow will be laminar or turbulent. Under turbulent conditions, heat transfer is maximized due to disturbance of boundary layers. This stems from the fact that heat transferred from the fin surface to the air is dependent on the thickness of the viscous region adjacent to the fin and the boundary layer, which in turn is dependent on the magnitude of the Reynolds number.
Today, HVAC/R systems are challenged to consume less energy and be more efficient in their operation. One means to accomplish this is the use of lower air speeds across the heat exchanger surfaces. Smaller motors consume less energy and produce a cost savings. Low airflow results in low Reynolds numbers, and laminar flow dominates. One method to overcome this performance deficiency is to increase fin density. Under such conditions, prior art louver designs are ineffective due to the suboptimal louver alignment between fins.
Acceptable fin structures must be both cost effective and efficient. However, often a more efficient design proves to be more expensive in terms of materials and/or manufacturing. Conversely, relatively simple designs tend to be less desirable because of inferior heat transfer characteristics. Therefore, a more efficient and economically viable heat exchanger fin design is sought. Also, it is desirable to minimize material costs by using thinner sheet metal, as conventional designs are already made with sheet metal which is as thin as practical.
Against this background, there is a need for an improved finned tube heat exchanger that achieves a lower air side pressure drop at a high fin density without sacrificing thermal performance.